gantry analysis

8/3/2019 Gantry Analysis

1/22Winter 2010 |PCI Journal

Editors quick points

n Precast concrete bridges are frequently built with self-launching

erection machines.

n Little has been written about these machines despite their cost,

complexity, and sophistication.

n This paper illustrates the main features of self-launching erec-

tion machines and presents some lessons learned.

Self-launching

erection

machinesfor precast

concrete

bridgesMarco Rosignoli

The technological aspects of construction influence the

modern bridge industry from the very first steps of design.

Entire families of prestressed concrete bridges, such as

launched bridges, span-by-span bridges, and balanced-can-

tilever bridges, take their names straight from the construc-

tion method.

Construction of precast concrete bridges with spans rang-

ing from 100 ft (30 m) to more than 600 ft (180 m) is

mostly based on the use of self-launching machines. The

launching units are complex and delicate structures. They

resist high loads on long spans under the same constraints

that the obstruction to overpass exerts on the final struc-ture. They are adaptable for reuse on different projects.

They must be as light as possible, which involves design-

ing for high stress levels in different load and support

conditions, and they are assembled and dismantled many

times and reused by different crews.

Little has been written on these machines in spite of their

cost, complexity, and sophistication. The present work

illustrates the main features of self-launching bridge erec-

tion machines and some lessons learned during 27 years

of the authors practice in the bridge industry and as an

independent design-checker of launching units.

Every construction method for precast concrete bridges

has its own advantages and challenges. In the absence of

particular requirements that make one solution immedi-

ately preferable to the others, the evaluation of the possible

alternatives is a difficult task. Comparisons based on the

quantities of materials consider only one of the compo-

nents of the construction cost of a bridge. In industrialized

countries, the cost of a bridge is more and more influenced

by the processing costs of the materials, such as labor, in-

vestments for specialty equipment, delivery and assembly

costs for the equipment, and energy.


2/22 3PCI Journal|Winter 2010

may be exposed to impacts and strong wind. The support

reactions are often applied eccentrically, the support sec-

tions are often devoid of diaphragms, and most units are

supported on deformable brackets or cross beams.

Mechanical and hydraulic components interact with the

structural components and often govern the stress distri-

bution. The safety of the unit itself depends on complexinteractions among mechanical, electrical, hydraulic, and

structural components. Indeed, such design conditions are

almost inconceivable in a permanent structure subjected to

such loads.

Types and featuresof launching units

The industry of self-launching machines is a specialty

niche. Every unit is originally conceived for a scope, every

manufacturer has its own technological habits, every con-

tractor has preferences and reuse expectations, and every

bridge has its own technical requirements. The length of

the bridge dictates the automation level of the equipment,

and even the construction country of the unit affects some

aspects of design. Nevertheless, there are not many con-

ceptual schemes.

The launching gantries for precast concrete girders com-

prise two parallel 3-D trusses. Two winch trolleys run

along the top chords with the girder suspended under-

neath, so no cross braces are typically installed between

the trusses. The span is relatively short; 150 ft (45 m)

spans are rarely exceeded in precast concretegirder

bridges, the design load of the unit is relatively low asthe girder is just a small portion of the span, and the

winch trolleys operate far from each other as the girder

is suspended at the ends. Therefore, these units are light,

deformable, and often comprise transportable modules

with tubular diagonals welded to the chords and through

pins at the field splices.

A launching gantry for span-by-span erection of precast

concrete segmental box girders operates on similar spans,

130 ft to 170 ft (40 m to 50 m), but the design load is much

higher because the unit supports the entire span during

segment assembly and the application of prestress.

The most versatile overhead units comprise two parallel

girders that suspend the deck segments and support the

runways for one or two winch trolleys. The girders are

supported by cross beams and are equipped with truss

extensions that control overturning during launching. The

winch trolleys operate along the entire unit so the main

girders are braced to each other only at the ends. The

girders comprise modules joined by pins or bolts, and the

modular nature of design often permits different assembly

configurations of chords and diagonals. These heavy work-

horse units are expensive on long bridges because of their

Savings in materials lower the construction cost of a bridge

only when they are not achieved with higher technological

costs. In other words, greater quantities of materials do not

necessarily make a solution uneconomical, provided that

the construction process is able to generate low labor costs

and to facilitate the amortization of specialty equipment.

A good balance of material costs and technological costsis the reason for the success of the incremental launching

method in industrialized countries.1 Compared with the use

of ground falsework, bridge launching diminishes the cost

of labor with similar investments in equipment. Compared

with the use of self-launching erection machines, bridge

launching diminishes the investments in equipment with

similar labor costs. In both cases bridge launching di-

minishes the technological costs, and even if the launch

stresses increase the cost of prestressing, the balance is

positive and the solution is financially effective.

The construction method that comes closest to bridge

launching is segmental precasting.2 Hundreds of bridges

have been built by segmental precasting even though the

need for avoiding joint decompression increases the cost of

prestressing. However, the investments in specialty equip-

ment are also high, so segmental precasting is typically

used for long bridges that allow amortization of precasting

facilities and erection machines. On shorter bridges, pre-

fabrication is limited to the concrete girders and the deck

slab is cast in place.

Precast concrete segments or girders can be erected with

ground cranes if the piers are not tall and the area under the

bridge is accessible. Sensitive environments, valleys withsteep slopes, tall piers, and inhabited areas often require

assembly by launching gantry, and in this case the techno-

logical costs increase significantly.

Self-launching erection machines are complex and delicate

machines. They resist huge loads on long spans, often the

weight of an entire span. Deck erection and self-launching

must be compatible with plan and vertical curvatures of the

bridge, and the most advanced units are also able to trans-

fer their support systems to avoid the use of ground cranes.

In spite of such complexity, the launching units mustalso be light. The weight governs the cost of the unit,

the delivery and assembly costs, and the launch stresses.

Weight limitation dictates the use of high-strength steel

and designing for high stress levels under different load

and support conditions.

The launching units are reused several times in different

conditions and by different crews. The units are modified

and adapted to new work conditions, field splices are as-

sembled and dismantled many times, and structural nodes

and splices are subjected to hundreds of load reversals.

The nature of loading is often highly dynamic and the units


3/22Winter 2010 |PCI Journal 3

The bridge itself may support lifting frames for precast

concrete segmental erection of balanced-cantilever bridg-

es. These light units are cost-effective for short bridges and

long spans, though they typically involve longer construc-

tion duration. Compared with a launching gantry, these

units also permit contemporaneous erection of several

hammers and erection sequences that do not require con-

struction from abutment to abutment.

Purpose-designed, multiwheel carriers with self-launching

support girders are used for transporting entire precast

concrete spans from the casting yard to the assembly loca-

tion along the completed bridge. The span length rarely

exceeds 130 ft (40 m) because of the prohibitive design

load for both the carrier and the bridge. Much longer

precast concrete spans can be handled with floating cranes

when the bridge dimensions permit amortization of such

investments.

Launching gantries forprecast concrete girders

The most common method for erecting precast concrete

girders is with ground cranes. Cranes usually entail the

simplest and most rapid erection procedures with the mini-

mum of investment, and the deck may be built in several

places at the same time. Good access is necessary along

the entire length of the bridge to allow the cranes to be

positioned and the girders to be transported and lifted into

place, and the bridge should be low to the ground. In the

presence of rivers, railroads, highways, or tall piers, crane

erection may not be possible. Ground transportation and

crane erection of precast concrete girders are often impos-sible in urban areas.

Ground gantries on tires or rails have been used to lift

the girders where level access is available and the deck is

very low to the ground. Less versatile than cranes, ground

gantries are typically used for erecting long urban viaducts

along existing roads. Paired gantries may be necessary

for the longest girders to limit the negative moment from

long-end cantilevers.

The use of a launching gantry often solves erection dif-

ficulties. A launching gantry for precast concrete girdersis a light modular structure comprising two parallel trusses

with triangular cross sections. The truss length is about

2.3 times the standard bridge span (Fig. 1). Light braced

frames support the trusses at the piers and allow the longi-

tudinal and transverse movements necessary to place the

girders with the due eccentricity and to launch the gantry

along curved alignments. The gantry operates without any

contact with the deck so the girders for many spans can be

placed before casting the deck slab.

Two winch trolleys run along the upper chords of the gan-

try and lodge two winches each. The main winch suspends

weight, the high labor demand, the complexity of opera-

tions, and the need for specific support structures.

Lighter and more automated, custom-designed, single-

beam units are often preferred on long bridges. These units

are based on one central beam; a light front extension

controls overturning during launching, and a rear portal

frame is supported by the new span. The front end of themain girder may also be supported by a self-launching

beam, and the connection may be pivoted to fit tight plan

curvatures. These units are lighter than the twin-upper-

beam units and do not require cross beams at the piers;

however, they are less versatile and adaptable. These units

are also more stable and compact and typically have full

self-launch capability.

Two parallel girders are also used in the underslung units.

The girders are typically equipped with front and rear ex-

tensions that control overturning and are supported on pier

brackets. The most advanced units are able to move the

brackets to the new pier. Struts from foundations are also

used to support the unit in bridges not very high above the

ground. In this case, the bridge span can be longer.

A cross beam running along the new span may suspend

the rear end of the unit. This diminishes the number of pier

brackets needed and shortens the unit. The front ends of

the main girders may also be connected with a cross beam

sliding along a central self-launching support beam. These

units are short and able to operate on tight plan curvatures.

Although the depth of the main girders may cause clear-

ance problems at the abutments or when overpassinghighways or railroads and their length is often a problem in

curved bridges, the twin-lower-beam gantries for span-by-

span precast concrete segmental erection are typically less

expensive than the twin-upper-beam units.

The upper-beam units for precast concrete segmental

balanced-cantilever erection can be operated on spans that

often exceed 350 ft (100 m). Compared with the launching

units for span-by-span construction, the design load is less

because the segments are handled individually or in pairs

and no entire span is suspended from the unit. Top-slab

tendons in the deck resist the negative moment generatedby the segment weight, and temporary pier locks resist the

load imbalance when the deck is not continuous with the

pier.

The design-governing load condition for the unit is typi-

cally the negative moment from the long front cantilever,

so varying-depth trusses are sometimes preferred. Stay

cables deviated by an extension tower of the main support

frame solve the most critical cases. The load deflections

are significant, but this is rarely a problem.



The typical support block for the main trusses comprises a

lower group of rolls or skids that move transversely along

the support cross beam and an upper group of rolls that

support the bottom chord of the truss. A transverse pin

connects the two groups of rolls and allows the upper rolls

to follow the rotations of the main trusses and the gradi-

ent of the launch plane. Some support blocks are equipped

with lock systems for the main trusses to avoid involuntary

movements of the unit. Most bridges have a longitudi-

nal grade and the gantries are supported on low-friction

inclined planes, so the lock systems are critical for the

stability of the unit and safety of operations.

Launching gantriesfor span-by-span erectionof precast concretesegmental box girders

Three different techniques can be used for erecting a pre-

cast segmental box girder:

span-by-span assembly

balanced-cantilever assembly

progressive placement with the help of temporary

stays or props

With the span-by-span method, all of the segments for a

span are positioned before the prestressing tendons are

installed, and the complete span is lowered onto the bear-

ings. The balanced-cantilever method involves erecting

the segments as a pair of cantilevers about each pier, and

the segments are prestressed with deck slab tendons that

cross the entire hammer. With the progressive placement, a

lifting frame or ground crane raises and places the seg-

the girder, and a smaller translation winch acting on an

endless ring cable moves the trolley longitudinally along

the gantry. The endless ring cable is anchored to the oppo-

site ends of the unit and is kept in tension by lever counter-

weights. A third trolley often carries an electric generator

that feeds gantry operations. Motorized wheels can also

be used for translation. Vertical hydraulic cylinders may

replace the main winches when the girders are deliveredalong the completed deck.

The gantry operates in one of two ways depending on how

the girders are delivered. If the girders are delivered at the

ground level, the gantry raises them to the deck level and

places them onto the bearings. If the girders are delivered

at the abutment, the gantry is moved back to the abutment

and the winch trolleys are placed at the rear end of the unit.

The front trolley lifts the front end of the girder and moves

it forward along the unit with the rear end supported on

the ground transportation unit. When the rear end of the

girder reaches the rear winch trolley, the latter picks it up

to release the ground transportation unit.

The longitudinal movement of the gantry is a two-phase

process. Initially, the gantry is anchored to a pier and the

winch trolleys move the girder one span ahead. Then the

winch trolleys are anchored to the pier and their translation

systems launch the trusses to the next span. This sequence

can be repeated many times so that when the girders are

delivered at the abutment, the gantry can place them sever-

al spans ahead. When the bridge is long, moving the gantry

over many spans slows down the erection and it may be

faster to cast the deck slab as soon as the girders are placed

and to deliver the next girders along the completed bridge.

The launching gantries for precast concrete girders are

relatively inexpensive and easily adaptable in both length

and spacing of the main trusses. They are flexible, so

wedge sledges are necessary at the ends of the trusses to

recover the elastic deflection during launching. These units

are able to cope with variations in span length and deck

geometry, and because they are located above the deck,

they are generally unaffected by ground-level constraints

and the plan curvature of the bridge.

Support frames anchored to the pier caps hold a rail forthe lateral movements of the unit. The cross beam of the

support frames typically comprises two I shapes connected

by horizontal bracing and diaphragms. Box girders are also

used in light applications. The cross beams have lateral

overhangs to shift the gantry laterally for the placement of

the edge girders and to launch the unit when the bridge is

curved in plan (Fig. 2). The support legs of the pier frames

are located so as not to interfere with the girders (Fig. 1).

They are adjustable (typically, hydraulic cylinders for

geometry adjustment and screw legs for the structural sup-

port) to set the transverse rail horizontal when the pier cap

is inclined.

Figure 1. This light gantry is used or precast concrete girders. Photograph

courtesy o Comtec.



ments in one direction from the starting point, passing over

the piers in the process. The balanced-cantilever method

is mostly used on long spans while long viaducts with

shorter spans are better suited to the span-by-span method.

Progressive placement is rarely adopted.

Span-by-span erection is used for both simply supported

spans and continuous superstructures. The adjacent spansof continuous bridges are joined together with in-place

stitches that avoid propagation of the geometry tolerances of

short-line segmental match-casting. After the closure pour

has hardened, continuity prestressing tendons are installed

and tensioned. With span-by-span erection and epoxy joints,

a typical 130 ft (40 m) span is usually erected every two or

three days. With a twin-lower-beam gantry and dry joints, an

erection rate of up to one span per day is achievable.

Span-by-span erection is typically used for spans shorter

than 160 ft (48 m). For longer spans, balanced-cantilever

erection is often more cost-effective because of the lowercost of the gantry. Progressive placement is usually the

most time-consuming erection technique because of the

single work location; however, the specialty equipment can

be particularly inexpensive, especially when ground cranes

can erect the segments along the entire length of the bridge.

Upper- or lower-beam gantries are used in the span-by-

span erection to support a complete span of segments,

which are pulled together by prestressing bars during

gluing of the joints and then by the permanent tendons.

The gantry then releases the span onto the bearings and

launches itself forward to erect the next span.

A typical twin-upper-beam gantry comprises two paral-

lel trusses or box girders supported on cross beams. The

truss units are preferred in high-wind regions and are

often lighter, while the box-girder units are more stable

and solid. The twin-upper-beam units are easily adaptable

to different span lengths, and they are able to cope with

variations in deck geometry. Because the main girders are

located above the deck, these units are less affected byground constraints; however, they are more complex to

design, assemble, and operate and the units are slower in

erecting the segments than an underslung gantry.

Overturning during launching is typically controlled with

extension trusses applied to the main girders. The total

length of the unit thus becomes about 2.3 times the stan-

dard span of the bridge, but this is rarely a problem with

overhead units. Cross beams anchored to the piers sup-

port the main girders with saddles that permit longitudinal

launching and lateral movements of the unit. The support

legs of the cross beams are adjustable to ensure that theframe is level. Hydraulic cylinders are used to adjust the

elevation, and safety ring nuts lock the cylinders during

operation and launching. The cross beams are anchored to

the pier cap with prestressing bars that resist uplift forces.

The cross beams have lateral overhangs to set the gantry

with the appropriate eccentricity (Fig. 3) and to launch the

unit on curved spans, so significant uplift forces may arise

in the anchor systems.

The support rollers comprise a lower group of transverse

rolls, which are supported on the cross beam, and an

upper group of longitudinal rolls that support the bottom

Figure 2. The edge girder is being placed. Photograph courtesy o Comtec.



chord. A transverse pin between the two roll assemblies

makes the support adaptable to rotations in the main gird-

ers and to launching onto grades. Some support blocks

lodge longitudinal lock systems for the unit, and all thesupport cross beams are typically equipped with trans-

verse lock systems.

The overhead gantries operate in one of two ways depend-

ing on how the deck segments are delivered. If the seg-

ments are delivered along the completed deck, a winch

trolley picks them up at the rear end of the gantry, moves

them over the span until reaching the assembly location,

and lowers them down to the deck level. If the segments

are delivered at the ground level, the winch trolley raises

them up to the deck level. Hangers are used to align and

hold the segments in position during assembly. Afterreaching the assembly location, the segments are hung

from the main girders and the winch trolley is released for

a new cycle. To avoid interference with the hangers of the

previously placed segments, the segments are moved out

with the long side in the longitudinal plane of the bridge.

The segments are rotated 90 deg just before suspension

with a special hook that is able to hydraulically turn the

segment.

Typically, all of the segments for the span are suspended

from the gantry before the joints are glued so that no ad-

ditional truss deflections can occur. Epoxy is applied to

groups of segments that are then pressed together with

temporary clamping bars. The permanent tendons are

usually tensioned from a stressing platform attached to the

front segment.

The lightest gantries may be launched with winches, such

as those units for precast concrete girders. Typically, how-

ever, long-stroke hydraulic cylinders lodged into the sup-

port saddles and acting against racks anchored to the main

girders are used to push the unit forward. Twin cylinders

are often used so that one cylinder anchors the unit during

repositioning of the adjacent cylinder. Figure 4 shows the

launch cylinders of a twin-lower-beam gantry in the raised

configuration for segment assembly. Lowering the support

jacks releases the span onto the launch bearings in one

operation.

Single-upper-beam gantries may also be used for span-by-

span precast concrete segmental erection. In these units,

the carrying structure is a longitudinal girder that is sup-

ported at the front pier of the span to be erected and at the

rear pier (in the case of simply supported spans) or on the

front overhang of the completed deck (in the case of a con-

tinuous bridge). The main girder may comprise two braced

trusses or plate girders, or it may be a triangular truss with

one upper chord and two bottom chords. A winch trolley

runs along the unit and moves the deck segments to the

assembly locations.

Figure 3. This shows a twin-upper-beam gantry with support cross beams. Photograph courtesy o NRS.



A light front extension typically controls overturning dur-

ing launching. The rear end of the unit is supported by the

completed span. No rear nose is necessary, so these units

are shorter than a typical twin-upper-beam gantry and are

more adaptable to curvatures in the bridge. The main girder

is stiffer than two parallel trusses, and its support systems

are also stiffer.

Lateral bracing connects the trusses or plate girders

along their entire length (Fig. 5). Lateral bracing typi-

cally includes cross beams between the flanges or chords,

connections designed to minimize displacement-induced

fatigue, and sufficient flexural stiffness to resist vibration

stresses. Cross frames connected to the flanges or chords at

the same locations of lateral bracing distribute torsion and

provide transverse rigidity. Connections are often designed

to develop member strength.

The gantry supports an entire span, so the main girder is

heavily stressed. The units for the heaviest spans are some-

times equipped with prestressing or stay cables, though

this complicates and slows the operations and increases

labor demand. Truss gantries are preferred in this case for

better control of buckling and simpler structural nodes at

the anchor points of stay cables.

Special support devices are necessary to launch the unit

when the front support frame is integral with the maingirder. Launching is typically achieved by friction, taking

advantage of the support reaction that the main girder

applies to the launcher. Figure 6 shows a typical friction

launcher assembled onto a support tower. A support box

is located underneath each bottom flange or chord of the

main girder. A hydraulic cylinder moves the box longitu-

dinally along the low-friction surface of a pivoted arm, and

two jacks at the opposite ends of the arm lift and lower the

main girder. The working sequence is as follows:

The vertical jacks lower the unit onto the support1.

boxes.

The thrust cylinders push the support boxes forward,2.

and the thrust force is transferred to the main girder by

friction.

When the launch cylinders reach the limit stop, the3.

jacks lift the unit and the support boxes may return

idle to the initial position to start this cycle again.

The friction launchers are typically pinned to the support

towers to allow rotations when launching along curved

alignments. Low-friction surfaces between the support

tower and the base frame also allow lateral shifting (Fig. 6).These geometry-control systems are equipped with sliding

clamps so that the entire assembly can be suspended from

the main girder (Fig. 7). Figure 7 shows the two friction

launchers of the unit suspended from the front overhang.

The rear support of the gantry is an adjustable frame that

runs along the completed span. Horizontal hydraulic cyl-

inders control the transverse alignment of the frame when

launching along curved spans, and vertical cylinders con-

trol the support reaction that the frame transfers to the deck.

Upon completion of span erection, the rear launcher is

moved backward to the front pier or overhang. In the firstphase of launching, the unit is typically supported at the

rear launcher and at the rear support frame. When the front

end of the unit reaches the next pier, the pier-cap segment

and the front launcher are positioned for launch comple-

tion. A front support leg typically controls overturning

during this operation. At the end of launching, the two

launchers are suspended from the front overhang and a

new span can be erected with the unit supported at the

main frame and the rear portal frame.

Refined single-upper-beam machines have been designed

for erecting precast concrete segmental box girders with

Figure 4. The launch cylinders o a twin-lower-beam gantry are shown in the

raised conguration or segment assembly.

Figure 5. Lateral bracing o a single-upper-beam gantry in a movable scaoldingsystem conguration connects the plate girders along their entire length.



ing noses after erecting the first span and launching the

unit to the second span. The front nose is also dismantled

before launching the unit to the last span. The abutment

walls must also be taller than the total depth of the unit so

as not to prevent operations in the first and last span.

The length and weight of the precast concrete segments for

box-girder bridges are usually governed by handling and

transportation requirements. Lengths up to 12 ft (3.6 m)

are often transportable on public roads without excessive

restrictions. If the precasting plant is close to the erection

site and no transportation restrictions exist, the segments

are made as long as practical, but they rarely exceed

lengths of about 14 ft (4.3 m).

To further increase the segment dimensions, a box girder

may be divided transversely into two halves to be joined

with in-place stitches in the two slabs. The girder may also

be divided into a pier-cap segment and a midspan segment

tight plan curvatures. In the unit ofFig. 8, the winch trolley

is suspended from the bottom flanges and the deck seg-

ments are delivered on the ground or along the completed

deck through the rear support frame. To accommodate

tight plan curvatures, the gantry comprises two elements:

a rear main girder and a front support beam. A turntable

with hydraulic controls for translation and vertical and

horizontal rotations connects the main girder to the supportbeam. During launching, the turntable pulls the main girder

along the support beam. When the front support frame has

reached the front pier, the support beam is launched for-

ward to clear the area under the main girder for assembly

of the new span.

Many precast concrete segmental bridges have also been

erected with underslung gantries. These units are posi-

tioned beneath the deck with the two trusses or box girders

on opposite sides of the pier, and the gantry supports the

segments under the lateral overhangs. The unit is typically

supported on pier brackets or props from foundations.

When the deck is low to the ground, midspan props may be

used to increase the operating span of the unit.

When overturning is controlled with front and rear exten-

sions, the length of the unit is more than twice the typical

span length. A central front launching beam may be used

in curved bridges to support the main girders in combina-

tion with a rear support frame running on the completed

deck. This type of gantry is a telescopic assembly of a

central support beam and two lateral girders that support

the segments. This solution requires a particular design of

the pier head to create the launch clearance for the support

beam.

The segments are placed onto the gantry with a crane or

lifting frame. When the segments are delivered through the

completed deck, the lifter is placed at the rear end of the

gantry. When the segments are delivered on the ground, the

crane is placed at the front end of the gantry. The segments

are placed onto the gantry close to the lifter and are moved

along the gantry to the assembly position with rollers.

Upon completion of assembly and application of prestress,

the gantry lowers the span onto the bearings.

Underslung gantries are simple to design, assemble, andoperate. Segment erection is fast, and props can be used to

extend the operating span when working low to the ground.

However, these units are not suitable for decks on a tight

horizontal curve. Vertical hinges in the main girders have

been used though the joints are heavily stressed and the

units are more complex to operate.

The underslung gantries also project beneath the deck,

which may cause clearance problems when passing over

existing roads or railroads and difficulties in the end spans

because the abutment walls are broader than the piers. This

problem is typically solved by assembling the rear launch-

Figure 6. A riction launcher is assembled onto a support tower.

Figure 7. The riction launchers are suspended rom the main girder.



The design-governing loading condition typically occurs

during handling of the pier-cap segment. This segment

is heavier and it is suspended at the center because it is

designed for negative moment, so the two winch trol-

leys work closely to each other and loading of the main

trusses is localized (Fig. 9). The segment weight is also

unevenly distributed between the winch trolleys to con-

trol overturning during insertion under the front support

cross beam. The lighter midspan segments are suspended

at the ends so that the winch trolleys work far from each

other, and the load displacement along the gantry is also

shorter.

The main trusses and the support cross beams are heavy,

so the launch stresses are also demanding on such long

spans. The weight of cross beams discourages crane

erection, and the gantry is typically able to reposition itssupporting systems. In this case, the unit has four support

points:

two main cross beams

a front arm used during transfer of the front cross

beam to the next pier

the pivoted rear leg, which is lowered behind the mac-

rosegment after its insertion under the gantry

The cross beams are moved forward with the winch trol-leys. Both cross beams usually lodge hydraulic cylinders

for the launch of the main trusses. The launch of a twin-

upper-beam gantry for macrosegmental construction is a

complex operation because these units are heaviest and the

load that they transfer to the bridge requires precise sup-

port locations. Control of overturning may require placing

the winch trolleys at the rear end of the unit and suspend-

ing counterweights, which further increases the launch

stresses and generates specific conditions of out-of-plane

buckling.

with in-place stitches at the span quarters. A macroseg-

mental span thus typically comprises four precast concrete

segments.

The macrosegments are transported longitudinally. The

segment length rarely exceeds 150 ft (45 m), so the maxi-

mum span length of the span-by-span macrosegmental

bridges is about 300 ft (90 m). In this case, the deck typi-

cally has varying depth. The segment weight is excessive

for ground cranes and also for most gantries for balanced-

cantilever construction, so special twin-upper-beam units

are used for macrosegmental erection (Fig. 9).

The segments are transported along the completed deck.

The length and weight of the segments are such that the

gantry cannot rotate them, so the segments are delivered

with their final alignment. The complexity of the opera-

tions and the cost of gantry are such that macrosegmentalconstruction is typically used for long parallel bridges low

to the ground. The four macrosegments for a span can thus

be supported on temporary towers at the front span quarter

before stitching and completion of prestressing. The rear

ends of the midspan segments are typically suspended from

the front overhang of the completed deck.

The launching gantries for macrosegmental construction

are subjected to specific design constraints. The segments

are very heavy, with the pier-cap segment typically heavier

than the midspan segment, and their weight may exceed

1500 kip (6.5 MN). The length and weight of segmentsprevent their being picked up from cantilever sections of

the gantry. Therefore, a rear pivoted leg is necessary. Com-

pared with a gantry for balanced-cantilever construction,

however, the front cantilever is shorter, so the total length

of the unit is similar. The pier-cap segments are inserted

longitudinally between the pier and the front support cross

beam, so the latter is supported on tall braced columns

(Fig. 9). The segments are also moved laterally, which

requires shifting the gantry along the support cross beams

and also shifting the winch blocks along the winch-trolley

cross beams to reach the maximum eccentricity. Overload-

ing the main trusses is increased further in curved bridges.

Figure 8. This is the pivoted single-upper-beam gantry or curved precast concrete

segmental bridges. Photograph courtesy o Deal.Figure 9. This heavy twin-upper-truss unit is or precast concrete macrosegmental

erection.



In a precast concrete segmental bridge, the pier-head seg-

ment should have the same weight as the other segments

so as not to require special lifting devices. The segment

contains a thick support diaphragm (the bearings are usually

eccentric with respect to the webs because of their dimen-sions) and the bottom slab is also thick because of the

longitudinal compression forces from the cantilevers, so the

pier-head segment is usually very short. This also facilitates

its placement because the gantry also must be supported at

the pier. Sometimes it is necessary to transfer some bending

to the piers by means of two parallel lines of bearings. Full

continuity can also be achieved by casting the pier-head seg-

ment in-place with through reinforcement from the pier.

The most common methods for precast concrete segmen-

tal balanced-cantilever erection are with ground cranes

or launching gantries. Ground cranes require access tothe deck along the entire length of the bridge. Ground

improvement may also be necessary. Cranes usually give

the simplest and most rapid erection procedures with the

minimum of temporary works. Cranes are also readily

available, and multiple cantilevers can be erected at once.

The main constraint on crane erection is access because

balanced-cantilever bridges are often selected in response

to inaccessible terrain.

Lifting frames operating on the deck are sometimes used

on tall piers or cable-stayed bridges. They are also selected

to use over water, where a custom-built system can accom-

Launching gantriesfor balanced-cantileverconstruction

Balanced-cantilever erection is a construction method wellsuited to precast concrete segmental bridges. The deck is

erected from each side of the pier in a balanced sequence

to minimize the load imbalance on the pier. This method is

particularly advantageous on long spans and where access

beneath the deck is difficult.

Segment assembly with ground cranes or lifting frames

permits free erection sequences, while the use of a launch-

ing gantry requires that the deck be erected from one

abutment toward the opposite one; however, this permits

delivering the segments along the completed deck and does

not require having access to the area under the bridge.

Balanced-cantilever bridges usually have box-girder

sections.3 Ribbed slabs have also been built in the past;

nowadays they are used almost only for cable-stayed

bridges, where most of the negative moment is resisted

by the stay cables. The deck can have constant or varying

depth. Constant-depth decks are easier to build, but they

are competitive in a narrower range of spans, 200 ft to

230 ft (60 m to 70 m). It is possible to erect a 130 ft (40 m)

precast concrete segmental span with epoxy joints in 3

days, while spans of 330 ft (100 m) typically take from 7 to

12 days to erect.

Figure 10. This long twin-upper-beam gantry is used or balanced-cantilever erection. Photograph courtesy o HNTB archive.



modate heavier segments. The pier-head segment is cast

in place to establish a platform on which one or two lifting

frames are secured. An auxiliary frame may also be used to

lift the pier-head segment and then the main lifting frame.

When a single lifting frame is used, the unit is moved from

one side of the pier to the other to lift the segments in turn.

Some lifting frames are able to pick up the segment at the

base of the pier and to move it along the cantilever. Usinga pair of lifting frames, one on each cantilever, simplifies

the erection process. Unless the bridge is very high off the

ground, lifting frames are used less than ground cranes

because of their cost and relative slowness and the disrup-

tion when moving to the next pier.

Launching gantries can speed erection rates, and when

the segments are delivered along the deck, site disruption

beneath the deck is minimized. They are suited to building

over rivers or other obstructions, though they are limited to

erecting the deck in a sequential manner and are delayed if

problems occur at any pier or span. Both single- and twin-

upper-beam units can be used. The gantry takes support

at the front pier of the span to be erected and on the front

overhang of the completed deck (Fig. 10).

The earliest gantries were slightly longer than the span to

be erected. The length was sufficient to span between the

front overhang of the completed deck and the next pier,

and minimizing the distance between the supports resulted

in a lighter gantry. Disadvantages of short gantries include

overloading the front deck overhang and the complexity

of placement of the pier-head segment and of the launch-

ing operations. The length of the most recent gantries is

typically twice the span length. These units take support

onto the piers, and the higher cost of the gantry is offset byless reinforcement and prestressing along the entire length

of the bridge. Placement of the pier-head segment and

launching are also simplified.

One or two winch trolleys transport the segments to the

assembly location. If the segments are delivered along the

completed deck, the winch trolley picks them up at the rear

end of the gantry and moves them out over the span. If the

segments are delivered at the ground level or on barges,

the winch trolley raises them up to the deck level.

A typical sequence for gantry erection is as follows:

The gantry places the pier-head segment onto the next1.

pier. At this stage, the front support frame of the gan-

try is on the pier and the central and rear support cross

beams are on the completed deck.

Figure 11. This single-upper-truss gantry is equipped with a deviation tower. Photograph courtesy o HNTB archive.



flange. The field splices in the chords are arranged with lon-

gitudinal bolts above the upper flange and below the bottom

flange to permit the wheels to pass through.

Macrosegmental construction is also compatible with

balanced-cantilever erection of long deck segments de-

livered on the ground. The weight of segments suggests

strand jacking for lifting as in Fig. 12. The segments are

connected with in-place stitches with through reinforce-

ment, and after application of prestressing the two seg-ments are released and the unit is ready for lifting another

pair of segments. This type of launching units can easily

be adapted to in-place casting (the lifting platforms are

replaced with shifting casting cells) and vice-versa.

The use of these heavy lifting units is suitable for spans

up to 350 ft to 400 ft (100 m to 120 m) and rectilinear or

slightly curved bridges of adequate length. The length of

the girders is about 1.3 times the maximum span length.

Despite their cost, these units offer many advantages. The

girder provides easy access to the work locations from the

completed deck for workers and materials. If the deck issupported onto bearings, the girder balances the cantilevers

without the deck being temporarily anchored to the pier.

This balancing action is also useful when the deck is con-

tinuous with the piers and the piers are tall and slender.

Wheeled carriersfor full-span precasting

Several major bridges have been built with concrete spans en-

tirely cast off-site and transported into place. Box girders are

well suited to highway and railroad bridges, while U-girders

are used only for railroad bridges, where they meet structural

The gantry moves its central support cross beam for-2.

ward onto the pier-cap segment.

The gantry is launched forward until it is sitting sym-3.

metrically above the pier.

After the gantry is anchored, the deck segments are4.

picked up and moved into position. New segments areplaced on either side of the pier and fixed with epoxy,

temporary joint clamping bars, and top-slab tendons.

When the two cantilevers are completed, aligned, and5.

locked to each other, a 1 ft to 2 ft (0.3 m to 0.6 m) in-

place closure segment is cast at midspan. Bottom-slab

tendons are installed across the joint to complete the

connection.

The pier-head segment is typically placed on temporary

supports while it is set to the correct alignment. When the

deck is supported on one line of bearings, the cantilevers

are stabilized with temporary lock systems. Props and

tie-downs from foundations are used with short piers. For

taller piers, stability is achieved with brackets or tie-down

arrangements comprising hydraulic jacks and vertical

prestressing bars. The temporary pier-head lock systems

are typically designed for a maximum of one segment out

of balance. Sometimes two segments are erected simul-

taneously on either side of the pier to reduce the load

imbalance, though this requires gantries equipped with two

winch trolleys. In some cases the gantry itself has been

used to stabilize the cantilevers.

Single-upper-truss gantries are sometimes equipped witha deviation tower at the central support and symmetrical

stays that relieve the stresses in the truss (Fig. 11). The rear

support leg is typically close to the rear pier to diminish

the stresses in the front deck overhang. This solution is

sometimes also adopted for span-by-span erection. In this

case, the deviation tower is placed at the rear pier and the

cable-stayed truss suspends the entire span.

The deviation tower is integral to the main support legs and

the truss. In the transverse plane, the tower is an A-frame

with the anchor section of stay cables at the top, the truss

at the middle, and two support legs that allow the precastconcrete segments to pass through. A base cross beam often

resists the horizontal forces generated by the leg inclination.

Truss towers and legs may be used to enhance rigidity.

A single plane of cables is generally preferred. The use of

numerous small cables facilitates their anchoring, and a fan

layout simplifies pull adjustment from a work platform. A

single support plane results in high torsion in the main truss,

however, so a few cables may be anchored to the bottom

chords to provide a torsional restraint. The winch trolleys

are typically suspended from the bottom chords; H-shapes

are used for the chords, and the wheels run onto the bottom

Figure 12. Strand-jack liting o macrosegments is used or balanced-cantilever

construction. Photograph courtesy o Thyssenkrupp.



and noise-containment requirements. Truss box girders with

one or two railroad tracks on the bottom slab and a roadway

on the top slab have also been entirely precast.

Full-span precasting results in rapid bridge construction

and repetitive casting processes in factory conditions. The

precast concrete units may be longer than 330 ft (100 m)

when floating cranes are used for placement. Groundtransportation is rarely adopted for spans longer than 170 ft

(50 m).

The precasting plant is usually located near the bridge site

to facilitate the transfer of the units. The units are removed

from the forms as soon as the concrete has reached the

required strength and are stored on temporary foundations

for completion of prestressing, application of bearings, and

finishing. The units are moved around the precasting plant

and storage areas with heavy gantries or wheeled carriers.

Rail-mounted gantries are often simpler to operate, while

wheeled carriers provide more flexibility. The reinforce-

ment cage is typically prefabricated complete with bulk-

heads and inserted into the casting cell by the heavy lifters

(Fig. 13).

For viaducts over land, the spans are transported along the

completed deck with special multiwheel carriers. The car-

rier ofFig. 14 was custom designed for erecting 755 spans.

After transporting the 1660 kip (7.4 MN), 103 ft (31.5 m)

span to the front end of the bridge, the front trolley of the

carrier moves forward along the 250 ft (76 m) support

beam until reaching the lowering position. The support

beam is then launched forward to the next span to clear the

area under the carrier for lowering the span.

The precast concrete spans are usually placed on bearings

to simplify the erection process, but they may be made

integral with the piers with in-place concrete connections.

The units can also be joined together with in-place con-

crete stitches to form a continuous structure.

Design loads

The load combinations used in the design of self-launching

bridge erection machines are complex because of the dy-

namic nature of loading. Because most units are designed

in Europe, the discussion that follows is based on the

FEM-1.001 standard for heavy lifters.4 The load classifica-

tions and combinations can easily be adapted to different

standards.

The heavy lifters are grouped into classes in relation to the

tasks they perform during their service life. The classifica-

Figure 13. The preabricated cage or the entire span is being inserted into the casting cell.



tion determines the load-amplification factor to be used

for the design of the structural components, which varies

from C= 1.00 for A1-class units to C= 1.20 for A8-class

units. The class of a unit is determined by the expected

number of load cycles and the loading level. A load spec-

trum relates the entity of loading to the number of cycles,

and since most load cycles of these units take place at or

near the load capacity, the spectral factor is typically high.

However, the number of cycles is low, so these machines

are often designed as A2-class units with load factor C=1.02.

A similar classification applies for the mechanical compo-

nents. The load-amplification factor varies from M= 1.00

for M1-class units to M= 1.30 for M8-class units, and

the bridge erection machines are often designed as M2- or

M3-class units with M= 1.04 and M= 1.08, respectively.

In both cases, the amplification factors are applied to the

individual loads, and the results are then processed with

the load factors for limit-state assessment prescribed by the

design standard.

The structural components are designed for static stresses

in the least favorable load conditions, inertial forces gener-

ated by vertical and horizontal movements of the load or

the unit, and meteorological loads. The design loads are

divided into four groups:

forces that act regularly during the normal operations

of the unit

forces that arise occasionally in the unit in service

exceptional forces in service and out-of-service condi-

tions

forces that arise during assembly and dismantling

The regular forces include self-weight, service load, and

inertial forces generated by load movements. The structural

weight resulting from cross-sectional areas is typically in-

creased by 30% to 40% to account for attached components,

such as stiffeners and connections. The accuracy of correc-

tion may be checked by weighting modules of the machine

Figure 14. This wheeled span carrier has a sel-launching support beam or high-speed railway projects.



during assembly. Concentrated loads represent specific

components, such as rollers and electric generators.

Service load includes lifted accessories, such as a gin

block, hook, positioning transoms, and suspension bars.

Vertical inertial forces derive from the sudden application

or removal of the load and from the positive or negative

accelerations during the vertical movements. The interven-tion of the emergency brakes in the case of an electrical

blackout may be analyzed with dynamic analysis and

assessed like any other service-limit-state (SLS) condition,

though the elasticity of such demultiplied ropes diminishes

the dynamic response significantly. The longitudinal iner-

tial forces derive from the accelerations or decelerations of

the winch trolleys along the runways.

The occasional forces are impacts of the wheels of the

winch trolleys against the runways, wind, snow, ice, and

thermal differences. The impacts along the runways are

often disregarded when the field splices in the rails are

welded and grinded away at dismantling. Snow and ice are

often disregarded in ordinary weather. FEM-1.001 requires

designing the erection machines for temperatures from

-4 F to 113 F (-20 C to 45 C).4

The exceptional forces are out-of-service wind, load test-

ing, impacts against end-of-stroke buffers or fixed obsta-

cles, and the design earthquake. Impacts against buffers at

the ends of the winch-trolley runways are often disregarded

for translation velocity lower than 2.3 ft/sec (0.7 m/sec),

provided that the runways are equipped with end-of-stroke

switches. The effects are computed in relation to the de-

celeration imposed on the winch trolley. The static stressesare often increased 25% for linear-spring buffers and 60%

for constant-force hydraulic buffers.

The design loads are grouped into three load conditions.

Load condition 1 is the normal operational condition,

which combines self-weight, superimposed dead load,

service load inclusive of vertical dynamic amplification,

and service load plus weight of winch trolley multiplied by

the longitudinal-dynamic-amplification factor. All of these

loads are then multiplied by the load-amplification factor

resulting from the classification of the unit. Load condition

2 is the operational condition in the presence of occasionalforces. It combines the actions of condition 1 with wind in

service, snow and ice, and thermal differences. In the case

of strong design wind, the longitudinal dynamic amplifica-

tion can be different from the value for condition 1 because

the action of wind can affect the starting and braking times

of the winch trolleys. Load condition 3 is the action of

exceptional loads. It considers the following combinations:

self-weight, superimposed dead load and out-of-

service wind

self-weight, superimposed dead load, service load, and

impacts against the end-of-stroke buffers

self-weight, superimposed dead load, service load and

design earthquake

self-weight, superimposed dead load, wind in service,

and assembly and dismantling operations

Although conceptually similar to assembly and disman-

tling, self-launching is typically assessed in the more

demanding condition 2 because of the higher frequency of

these operations. Load testing typically requires specific

checks when the static test load is greater than 140% of the

design load or the dynamic test load is greater than 120%.

Both SLSs and ultimate limit states (ULSs) are assessed.

SLSs correspond to the loss of functionality of the unit

and are related to internal displacements of components or

rotations of slip-critical connections. Unit load factors are

applied to the three load conditions, and the resistance fac-

tors are prescribed by the steel design standard. The ULSs

correspond to critical conditions, such as rigid equilibrium,

rupture of connections, yielding of structural elements,

and local buckling. The three load conditions are handled

with progressively lower load factors. According to CNR-

10021,5 for example, it is LC - 1 = 1.50, LC - 2 = 1.34, and

LC - 3 = 1.20.

Most design standards do not distinguish between local

and global (out-of-plane) buckling, and both conditions

may be assessed like any other ULS condition. However,

out-of-plane buckling of long sections of compressionchords is a riskier event than local buckling of a web panel

or a secondary compression member. No post-critical

domain exists in most cases, and the critical load is influ-

enced by geometry imperfections that are difficult to de-

tect. It is therefore common practice to assess out-of-plane

buckling with higher load factors, LC-1 = LC-3 2.5.

Modeling and analysis

The optimum level of detail for the finite-element (FE)

model of a launching unit is always a major concern. The

more refined the model, the more accurate the results ofanalysis. Alternatively, the number of load and support

conditions to analyze suggests using simple models to

rapidly investigate all of the possible combinations. Simple

beam models facilitate the research of the design-gov-

erning load conditions and provide the stress magnitude

to be expected from more refined analyses as well as the

boundary conditions to assign to local models. Afterward,

however, the different types of bridge erection machines

require specific approaches to FE modeling and analysis.

The twin-lower-girder units are typically supported on

stiff pier brackets, and simple beam models often suffice.



cross beams and P-delta effects in the support towers. It is

therefore necessary to check that the longitudinal stiffness

of the support points of the main girders is higher than the

breakaway friction of sliding bearings.

Instability of main trusses

Among the factors influencing the stability of a freestand-ing truss are the degree of fixity at the supports, a support

condition prompting the truss to twist as it deflects later-

ally, the lateral restraint exerted by the inclined diagonals

on the compression chords, the location of concentrated

loads, and the level of imperfections in the initial geometry

of the truss.

In the twin-truss units, these factors coexist and coalesce.

The degree of fixity at the supports is low, the truss being

supported at the bottom chords on flexible brackets or

cross beams with out-of-node eccentricity and without

cross diaphragms. The truss bends laterally and twists

because of the deflections in the support cross beams, and

the height of the truss amplifies the lateral displacements

of the upper chord. The truss is typically high and narrow,

so the lateral restraint that the inclined diagonals exert

onto the upper chord is poor. The winch trolleys run along

the upper chord, so the load is applied above the center of

shear. Finally, the geometric imperfections may be signifi-

cant because of the high number of field splices and the

tolerances accumulated in second-hand equipment.

The design standards normally recognize two types of

instability; the first type is related to the overall sway of

the structure (out-of-plane buckling) and the second type isrelated to deformations of a compression member between

its end nodes (local buckling). Out-of-plane buckling can

rarely be assessed with bibliographic values of the critical

elastic moment in such complex structures. Investigating

many buckling modes with complex numerical models in-

volves long computational times, so only the first modes of

out-of-plane buckling are typically analyzed numerically,

and the local modes are checked with the load-magnifica-

tion factors prescribed by the design standard.6

Linear buckling analysis investigates the stability of a

structure under a specified set of loads. The buckling modesdepend on the load, and instability must therefore be as-

sessed for different load conditionsthat is, by reproducing

the placement of the design-governing deck segments. The

inertial load amplification also affects the buckling factor

during the vertical and lateral movements of load.

Excessive confidence with a launching gantry as a result

of having already handled similar loads in the past may be

a serious mistake. The stability of a gantry does not only

depend on the entity of the load but also on how the load is

applied to the unit. When a gantry handles a long precast

concrete girder, the winch trolleys are at the girder ends

Twin-lower-truss units are more complex, and 3-D models

are recommended when the pier brackets are flexible or

only some of the trusses support the deck segments. Mod-

eling the support structures is also necessary when the unit

is supported onto permanent piers and temporary props

from foundations as the different flexibility and thermal

inertia of supports affect load distribution and buckling

factors.

In a single-upper-beam gantry, the front support legs are

close to each other to be supported by the pier while the

rear legs are distant to feed the assembly area with precast

concrete segments through the completed deck. In the pres-

ence of so many technological requirements, the structural

nodes are so complex (Fig. 15) that local 3-D solid models

are often necessary. Simple beam models are used to

analyze span erection, the deck-gantry interaction at the

application of prestress, and self-launching.

When a twin-upper-truss unit is supported by deformable

cross beams, analyzing the unit as a single 3-D truss on

rigid supports leads to imprecise results. Modeling the en-

tire unit allows for evaluation of the effects of cross-beam

deflections.

An ideal truss should fulfill three basic conditions. The

members of diagonals and chords are perfectly hinged

at the truss nodes. This condition is never respected in a

launching unit. The truss nodes are continuous, and when

the unit is provided with assembly pins, these are typically

located far from the nodes. The loads are applied at the

truss nodes. Also this condition is never respected. Techno-

logical requirements dictate the location of the suspensionpoints of segments, and the load applied by winch trolleys

and support rollers migrates along the chords. The grav-

ity axes of all members converging into a node cross at

the geometric panel node. This condition could actually

be met, though in most cases the convergence points of

the diagonals onto the chords are eccentric to simplify the

design of nodes.

Depicting the stresses resulting from these geometry im-

perfections requires accurate modeling. The model should

describe the entire unit3-D trusses, support cross beams,

and support towers. Out-of-node eccentricities and steps inthe bending plane at the changes of cross section should be

considered, and end offsets should be used to account for

the finite size of diagonal and chord intersections.

The reliability of internal releases of degrees of freedom

should be critically reviewed. The twin-upper-beam units

are often supported on cross beams that are held up by

steel towers. The main tower provides the longitudinal

restraint, and the secondary tower may be equipped with

sliding bearings. Before overcoming breakaway friction,

the sliding bearings do not slide at all and thermal expan-

sion of the main girders generates horizontal bending in the



Figure 15. This shows the main structural node o an overhead gantry.



to reduce the risk of progressive collapse is to require

insensitivity to local failure. In other words, local buckling

of a primary load-carrying member must not cause col-

lapse of the gantry or of a major part of it.

Although the structural damage induced by local buckling

is limited, the sudden stress redistribution generated by the

loss of carrying capacity of a member is a highly dynamic

process that requires analysis in the time domain.6 The

resulting stresses can be assessed like any other ULS con-

dition. Dynamic amplification is often low thanks to theflexibility of these units, and because the support rollers

are long, several pairs of diagonals are typically involved

in the load path at supports.

Local buckling in the compression flange can trigger

critical situations in the box-girder units because during

launching the support sections are devoid of stiffeners and

diaphragms. In the unit in Fig. 16, the left girder (on the

right side in the photograph) was slightly misaligned left-

ward so that when the front overhang was already 157 ft

(48 m) long, the operator of the unit decided to realign it.

A cross beam was placed near the front support saddle to

and the load that they apply is one-half of the total weight.

When the winch trolleys operate near each other (for ex-

ample, in a heavy pier-cap segment), the total load may be

similar but the stresses in the compression diagonals may

be much higher.

Overloaded members may buckle suddenly, so careful

inspections are necessary during assembly and at regular

intervals during the use of a launching unit. Damaged

diagonals must be reinforced or replaced even when they

apparently are in noncritical locations because the loadconditions are so varying that they might become the criti-

cal element of the structure. The cross diaphragms should

also be inspected frequently.

The stability of a launching gantry depends on the stability

of members against local failure and on the unit response

to local failure. Local buckling of a primary load-carrying

member is often critical in the support towers, while stable

alternate load paths often exist in such redundant trusses;

however, local buckling may trigger a chain reaction of

failures causing progressive collapse. The simplest ap-

proach to ensure the robustness of a launching gantry and

Figure 16. The buckling o the compression fange at the supports is due to the wrong operation.



alignment required lateral shifting of the base hydraulic

cylinders of the support frame to align them with the

bridge webs, so the cross beam was windowed in its end

sections. As a result, the torsion constant in the two 12 ft

(3.6 m) windowed portions of the cross beam became

about one-thousandth of the original box-section constant.

When the support cylinders are at the ends of the crossbeam, the two vertical load paths are three-aligned-hinge

schemes where the central cylindrical hinge has minimal

rotational stiffness because of the low torsion constant of

the windowed section (Fig. 17). The scheme is more stable

when the support cylinders are under the inclined legs

of the W-frame, so buckling was likely to occur only in

curved bridges. The buckling factor was as low as LC - 1 =

0.6, and triangular box stiffeners were applied to increase

the torsional constant and stabilize the frame.

The design of connections has a fundamental influence

on the strength and stability of the support towers. When

the legs are always compressed and the contact between

column and end plate is machined, the welds at the ends of

the module are subjected to minimal stresses. In the case

of load reversal, therefore, the end welds are often de-

signed for tension and the much higher compression force

is resisted by the machined contact. In the case of roughly

machined contacts, however, the weld can break under the

compression force, and at load reversal, the column can

detach from the end plate.

Launch and lock systems

In light units such as lifting frames, the launch stresses arerelatively low and the operations are simple and intuitive.

In heavy units such as long gantries, the launch stresses

are so great that they often govern the design of primary

components of the unit. The launch stresses depend on the

launch procedures, and launching heavy units involves

sequences of operations that are typically more complex

the heavier the unit is.

Light gantries for erection of precast concrete girders and

many heavy units use assemblies of cast-iron rolls on

rocking arms for the launch bearings. The number of rolls

depends on the load and the diameter. It is usually two,four, or eight, with progressively higher costs. Lateral

guide may be ensured with rolls acting against the bottom

flange or a steel rail welded under the web.

The use of cable bearings is less frequent. The support

reaction is distributed by a tensioned ring cable that

directly supports the rolls, so these bearings are thinner

and more stable than the roll bearings with rocking arm.

When the launch occurs along a curve, cable bearings

may be combined with orientation ball-plate bearings and

shifting bearings, which respectively orient and align the

rolls under the webs. Cable bearings may also be placed

support two flat jacks on PTFE plates. The jacks were to

be inserted under the box-girder webs to then pull the jacks

and box girder rightward along the low-friction contact.

However, the procedure was misunderstood and after rais-

ing the box girder with the flat jacks, the PTFE plates were

inserted between the box girder and the main support jacks.

In the initial stage of realignment, the box girder resisted

the transverse bending generated by the increasing ec-

centricity in the support reactions. Eventually, the outer

flange and the central flange panel buckled upward. This

generated two low-friction inclined planes that gave rise

to uncontrolled rightward sliding of the box girder. Flange

buckling further increased, and both webs also buckled.

Collapse was avoided because the end block of the cross

beam stopped the box-girder stroke when the left jack (on

the right in the photograph) was already almost disengaged.

Support member instability

The weight of the gantry and the service load are trans-

ferred to the bridge foundations through complex load

paths that typically include adjustable components, hy-

draulic systems, and pivoted legs. These support systems

are affected by specific forms of instability. The pivoted

support legs, in particular, are among the most delicate

components of a launching gantry.

A gantry had a pivoted W-shaped rear support frame,

where the base cross beam originally had a box section

over the entire width. Reusing the gantry on a curved

Figure 17. This gure shows a rear portal rame with shiting support pistons.



the launch. Therefore, the equipment can be designed for

the launch loads and overloaded without excessive worries

if necessary. The electronic control of the hydraulic plant

permits synchronization of launchers and the ability to set

limit pressures avoids overloading. Launch-cycle automa-

tion with end-of-stroke switches facilitates the operations

and increases the launch speed.

When approaching the pier, the front end of the gantry

is deflected downward. In the lightest gantries, the front

deflection is recovered by inclining the bottom flanges

to force progressive realignment (Fig. 2). The alignment

force rarely overloads the support cross beams, but the

launch bearings must be anchored to avoid displacement

or overturning. The launch bearings must also be articu-

lated to accommodate large rotations. Realignment may

also be achieved with long-stroke hydraulic cylinders that

move steel arms hinged to the nose tip. When the lifting

arm reaches the pier, it is lowered down and forced until

recovery of the elastic deflection. Hybrid solutions are also

possible where the bottom chords are rounded and a front

hydraulic cylinder recovers only a portion of the cantilever

deflection.

Several overhead gantries are designed for launching their

own support systems. In this case, the front tip is often

equipped with a vertical arm that during launching takes

support at the front pier to stabilize overturning. After

forcing the front arm, the winch trolley places the pier cap

segment and then advances the front support cross beam

and places it onto the segment, and the launch can restart.

Load testing

Bridge erection machines are typically load tested upon

completion of the first assembly. New load tests and

comparisons with previous tests should also follow every

major reassembly of the unit.

Launching gantries are subjected to static and dynamic

tests. The static test load is 10% to 40% higher than the

design load of the unit. Load testing takes place in the ab-

sence of wind and consists of slowly lifting a progressively

increasing load until reaching the full test load. The load

is lifted at different locations to reach full design stressesin the critical components of the unit, the deflections are

compared with the theoretical values, and deflection re-

covery is also checked. The dynamic test load is typically

10% to 20% higher than the design load. The movements

of the gantry are tested individually at increasing velocities

until they reach the maximum values. Blackout tests can

also be performed to measure the dynamic response to the

intervention of the emergency brakes.

onto hydraulic jacks for accurate distribution of the support

reactions and the creation of torsional hinges.

Sliding bearings may also be based on polytetrafluoroethyl-

ene (PTFE) skids. The bottom flange typically slides along

lubricated PTFE surfaces without interposition of stainless-

steel sheets. Dispersal of the support reaction into the webs

may be facilitated with multilayered elastomeric blockscovered with a dimpled PTFE plate. These blocks are

aligned in rocking frames on ball-and-socket articulations or

transverse pins. At the current state of practice, roll bearings

and sliding bearings are complementary. Sliding bearings

are fit for slow launching of high loads, while roll bearings

are fit for medium loads and high launch velocities.

The launch and lock systems have a substantial impact

on the stability and safety of a launching gantry. Launch-

ing is achieved with one of three methods. In the simplest

and oldest units, a winch trolley is anchored to a pier and

its translation winch moves the main trusses to the next

span. In more recent units, long-stroke hydraulic cylinders

lodged into the support saddles push the unit forward by

acting against racks fixed to the bottom flanges. In the

most advanced units, the thrust force is transferred by

friction using hydraulic launchers that directly support the

girders.

The light gantries for precast concrete girders are usually

moved with winches, though this solution presents some

disadvantages, especially when launching along inclined

planes. The longitudinal grade of many bridges is close to

the friction coefficient of the rollers, so when launching

occurs with new or well-greased rollers it can be necessaryto brake the unit to prevent uncontrolled sliding. Braking

of any component of the tow system also leaves the unit

unrestrained on low-friction supports. This requires design

precautions and the adoption of higher safety factors,

which involves oversizing equipment and operating more

slowly.

Higher thrust forces require mechanical transmission by

means of launch cylinders acting against racks fixed to the

bottom chords, as in Fig. 4. These launch systems are very

efficient in the case of horizontal launching, and they also

permit backward pulling of the unit. When launching upgrade, the unit must be locked during the return stroke of

the launch cylinders to avoid uncontrolled backward slid-

ing. Two adjacent launch cylinders are used in this case.

In the more refined units, the launch force is transferred

by friction (Fig. 6). For the unit to be trailed, it is neces-

sary that the ratio of the thrust force to the support reaction

onto the launcher be smaller than the friction coefficient

between the two steel surfaces. The support reaction varies

during launching, so two synchronized launchers are gen-

erally used. Friction launchers offer high intrinsic safety

because the worst consequence of hydraulic faults is halt of



Conclusion

The bridge industry has seen unbelievable progress in the

past decades. New means of analysis and a better knowl-

edge of mechanics of materials have permitted new struc-

tural solutions and are at the basis of architectural wonders

that will represent our legacy, as a bridge community, to

the next generations. Such progress, however, is also theresult of the technological advance in the erection methods

for precast concrete bridges.

The technological aspects of erection will have a more and

more marked influence on the modern bridge industry, and

construction of precast concrete bridges is mostly based on

the use of self-launching machines. In spite of the technical

skill and the quality-assurance and quality-control pro-

cesses of many manufacturers of launching units, accurate

custom-written technical specifications and independent

checking of design may be precious tools in making better

and more circumstantiated decisions and dodging avoid-

able mistakes.

References

Rosignoli, M. 2002.1. Bridge Launching. London, UK:

Thomas Telford.

American Segmental Bridge Institute (ASBI). 2008.2.

Construction Practices Handbook for Concrete Seg-

mental and Cable-Supported Bridges. Phoenix, AZ:

ASBI.

Hewson, N. 2003.3. Prestressed Concrete Bridges:Design and Construction. London, U.K.: Thomas

Telford.

Federation Europeenne de la Manutention FEM 1.001.4.

1987.Rgles pour le Calcul des Appareils de Levage.

[In French.]

CNR 10021. 1985.5. Strutture di Acciaio per Apparec-

chi di Sollevamento. Istruzioni per il Calcolo, lEsecu-

zione, il Collaudo e la Manutenzione. [In Italian.]

Rosignoli, M. 2007. Robustness and Stability of Lau-6.nching Gantries and Movable Shuttering Systems

Lessons Learned. Structural Engineering Internatio-

nal, 02/2007: pp. 133140.


22/22

About the author

Marco Rosignoli, Dr.Ing., P.E.,

has 27 years of experience inbridge design and construction.

For 15 years he has served as

construction manager, project

manager, and bridge-department

lead for prime European bridge

contractors. After assisting contractors on complex

bridge projects in 19 countries and 4 continents for 9

additional years, he has served as chief bridge

engineer at HNTB Corp. since 2006.

Rosignoli has been the independent design checker of

many self-launching bridge-erection machines. Expert

in bridge construction technologies, he has written two

books and more than 60 publications. He is currently

chairing the working group WG-6, Bridge Construc-

tion Equipment, of the International Association for

Bridge and Structural Engineering.

Synopsis

Launching units are complex and delicate structures.

They resist high loads on long spans under the same

constraints that the obstruction to overpass exerts

onto the final structure. They are adaptable for reuse

on different projects. They must be as light as pos-sible, which involves designing for high stress levels

in different load and support conditions, and they are

assembled and dismantled many times and reused by

different crews.

For all of these reasons, self-launching bridge erection

machines are typically purchased or leased based on

purpose-written technical specifications, their design

is subjected to independent checking, load testing isfrequent at the end of assembly, and the operations are

ruled by written procedures.

Little has been written on these machines in spite of

their cost, complexity, and sophistication. This article

illustrates the main features of self-launching bridge

erection machines and includes some lessons learned.

Keywords

Bridge, girder, launching gantry.

Review policy

This paper was reviewed in accordance with the

Precast/Prestressed Concrete Institutes peer-review

process.

Reader comments

Please address any reader comments to PCI Journal

editor-in-chief Emily Lorenz at [email protected]

or Precast/Prestressed Concrete Institute, c/o PCI

Journal, 209 W. Jackson Blvd., Suite 500, Chicago,

IL 60606. J

gantry analysis

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